This invention relates to glass compositions that are adapted for formation into fibers that can be employed for reinforcing composite substrates comprising a printed circuit board (“PCB”). More particularly, the invention relates to glass fiber reinforcements that have electrical properties that permit enhancing performance of a PCB.
“Dk” is the dielectric constant of a material, also known as “permittivity” and is a measure of the ability of a material to store electric energy. A material to be used as a capacitor desirably has a relatively high Dk, whereas a material to be used as part of a PCB substrate desirably has a low Dk, particularly for high speed circuits. Dk is the ratio of the charge that would be stored (i.e., the capacitance) of a given material between two metal plates to the amount of charge that would be stored by a void (air or vacuum) between the same two metal plates. “Df” or dissipation factor is the measure of the loss of power in a dielectric material. Df is the ratio of the resistive loss component of the current to the capacitive component of current, and is equal to the tangent of the loss angle. For high speed circuitry, it is desired that the Df of materials comprising a PCB substrate be relatively low.
PCB's have commonly been reinforced with glass fibers of the “E-glass” family of compositions, which is based on “Standard Specification for Glass Fiber Strands” D 578 American Society for Testing and Materials. By this definition, E-glass for electronic applications contains 5 to 10 weight percent B2O3, which reflects recognition of the desirable effect of B2O3 on dielectric properties of glass compositions. E-glass fibers for electronic applications typically have Dk in the range 6.7-7.3 at 1 MHz frequency. Standard electronic E-glass is also formulated to provide melting and forming temperatures conducive to practical manufacturing. Forming temperatures (the temperature at which the viscosity is 1000 poise), also referred to herein as TF, for commercial electronic E-glass are typically in the range of 1170° C.-1250° C.
High performance printed circuit boards require substrate reinforcements having lower Dk compared to E-glass for better performance, i.e., less noise signal transmission, for applications in telecommunication and electronic computing. Optionally, reducing Df relative to E-glass is also desired by the electronic industry. While the PCB industry has a need for low dielectric fiber glass, manufacture of glass fiber reinforcement requires economical viability issues to be addressed in order for low dielectric fibers to achieve successful commercialization. To this end, some low Dk glass compositions proposed in the prior art do not adequately address the economic issues.
Some low dielectric glasses in the prior art are characterized by high SiO2 content or high B2O3 content, or a combination of both high SiO2 and high B2O3. An example of the latter is known as “D-glass.” Detailed information on this approach to low Dk glass can be found in an article by L. Navias and R. L. Green, “Dielectric Properties of Glasses at Ultra-High Frequencies and their Relation to Composition,” J. Am. Ceram. Soc., 29, 267-276 (1946), in U.S. Patent Application 2003/0054936 A1 (S. Tamura), and in patent application JP 3409806 B2 (Y. Hirokazu). Fibers of SiO2 and glasses of the D-glass type have been used as reinforcement in fabric form for PCB substrates, e.g., laminates comprised of woven fibers and epoxy resin. Although both of those approaches successfully provide low Dk, sometimes as low as about 3.8 or 4.3, the high melting and forming temperatures of such compositions result in undesirably high costs for such fibers. D-glass fibers typically require forming temperatures in excess of 1400° C., and SiO2 fibers entail forming temperatures on the order of about 2000° C. Furthermore, D-glass is characterized by high B2O3 content, as much as 20 weight percent or greater. Since B2O3 is one of the most costly raw materials required for manufacturing conventional electronic E-glass, the use of much greater amounts of B2O3 in D-glass significantly increases its cost compared to E-glass. Therefore, neither SiO2 nor D-glass fibers provide a practical solution for manufacturing high performance PCB substrate materials on a large scale.
Other low dielectric fiber glasses based on high B2O3 concentrations (i.e., 11 to 25 weight percent) plus other relatively costly ingredients such as ZnO (up to 10 weight percent) and BaO (up to 10 weight percent) have been described in JP 3409806B2 (Hirokazu), with reported Dk values in the 4.8-5.6 range at 1 MHz. The inclusion of BaO in these compositions is problematic because of cost as well as environmental reasons. In spite of the high concentrations of the costly B2O3 in the compositions of this reference, the fiber forming temperatures disclosed are relatively high, e.g., 1355° C.-1429° C. Similarly, other low dielectric glasses based on high B2O3 concentrations (i.e., 14-20 weight percent) plus relatively costly TiO2 (up to 5 weight percent) have been described in U.S. Patent Application 2003/0054936 A1 (Tamura), with Dk=4.6-4.8 and dissipation factor Df=0.0007-0.001 at 1 MHz. In Japanese Patent Application JP 02154843A (Hiroshi et al.) there are disclosed boron-free low dielectric glasses with Dk in the range 5.2-5.3 at 1 MHz. Although these boron-free glasses provide low Dk with presumably relatively low raw material cost, their disadvantage is that fiber forming temperatures at 1000 poise melt viscosity are high, between 1376° C. and 1548° C. Additionally, these boron-free glasses have very narrow forming windows (the difference between the forming temperature and the liquidus temperature), typically 25° C. or lower (in some cases negative), whereas a window of about 55° C. or higher would commonly be considered expedient in the commercial fiber glass industry.
To improve PCB performance while managing the increase in cost, it would be advantageous to provide compositions for fiber glasses that offer significant improvements of electrical properties (Dk and/or Df) relative to E-glass compositions, and at the same time provide practical forming temperatures lower than the SiO2 and D-glass types and the other prior art approaches to low dielectric glass discussed above. To significantly lower raw material costs, it would be desirable to maintain B2O3 content less than that of D-glass, e.g., below 13 weight percent or below 12 percent. It can also be advantageous in some situations for the glass composition to fall within the ASTM definition of electronic E-glass, and thus to require no more than 10 weight percent B2O3. It would also be advantageous to manufacture low Dk glass fibers without requiring costly materials such as BaO or ZnO that are unconventional in the fiber glass industry. In addition, commercially practical glass compositions desirably have tolerance to impurities in raw materials, which also permits the use of less costly batch materials.
Since an important function of glass fiber in PCB composites is to provide mechanical strength, improvements in electrical properties would best be achieved without significantly sacrificing glass fiber strength. Glass fiber strength can be expressed in terms of Young's modulus or pristine tensile strength. It would also be desirable if new low dielectric fiber glass solutions would be used to make PCB without requiring major changes in the resins used, or at least without requiring substantially more costly resins, as would be required by some alternative approaches.